U-PB TITANITE DATES CORROBORATE VERY SLOW EXHUMATION OF THE NORTH-EAST GREENLAND ULTRAHIGH-PRESSURE TERRANE FROM THE CALEDONIAN OROGEN

Quartzofeldspathic gneiss that hosts eclogite is the dominant lithology of the continental ultrahigh-pressure (UHP) terrane in the North-East Greenland Caledonides. Both lithologies contain coesite, indicating all the rocks reached mantle depths. The best-preserved gneiss is a clinopyroxene-bearing granulite with HP phases (kyanite, phengite, omphacite and rutile) included in garnet. Titanite is common in the retrogressed gneiss and shows a textural progression from overgrowths on matrix rutile to polycrystalline grains with no rutile to euhedral porphyroblasts. Titanite from 6 gneisses has been analyzed for U-Pb and trace elements by SHRIMP-RG using McClure Mountain titanite as a standard. Weighted mean ²⁰⁶Pb/²³⁸U dates, using data-defined common Pb values, range from 347 ± 6 to 321 ± 7 Ma. Regression of data from all samples defines an intercept of 331 ± 4 Ma (MSWD=2.6), which is taken as the age of rutile-to-titanite conversion. Four samples show concave-down trace element patterns, similar to igneous titanite, interpreted as formation in the presence of a melt; indeed, three of the samples contain thin, wispy leucosomes. Large (>600 µm), zoned subhedral titanite from a metagranitoid gneiss preserves three distinct trace element patterns that can be related to melting, metamorphism and garnet breakdown. Zr-in-titanite temperatures, calculated at 0.5 GPa and aSiO₂=aTiO₂=1, range from 690–723°C. The preservation of different trace element and age domains argues against simple cooling ages.

UHP ages from coesite-bearing zircon in eclogite and gneiss are ≈365–350 Ma, or approximately 35–20 m.y. earlier than titanite growth. Titanite from UHP gneiss in NE Greenland formed over a prolonged exhumation history also recorded by zircon: (1) Phengite decompression melting at the quartz-coesite transition starting at ≈347 Ma; (2) Granulite to amphibolite facies metamorphism accompanied by pegmatite emplacement between 350 and 340 Ma; and (3) Continued amphibolite facies exhumation with titanite growth through 320 Ma. This contrasts with many UHP terranes where titanite grows within < 4–15 million years of peak P. Slow exhumation agrees with a tectonic setting at the base of an overthickened, overriding plate where UHP occurs in intracontinental subduction zones, as has been proposed for present day Tibet.